Extension Scripts

Tom Eastep

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover, and with no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”.

2009/06/20


Table of Contents

Extension Scripts
Compile-time vs Run-time Scripts

Caution

This article applies to Shorewall 4.3 and later. If you are running a version of Shorewall earlier than Shorewall 4.3.5 then please see the documentation for that release.

Extension Scripts

Extension scripts are user-provided scripts that are invoked at various points during firewall start, restart, stop and clear. The scripts are placed in /etc/shorewall and are processed using the Bourne shell “source” mechanism.

Caution

  1. Be sure that you actually need to use an extension script to do what you want. Shorewall has a wide range of features that cover most requirements.

  2. DO NOT SIMPLY COPY RULES THAT YOU FIND ON THE NET INTO AN EXTENSION SCRIPT AND EXPECT THEM TO WORK AND TO NOT BREAK SHOREWALL. TO USE SHOREWALL EXTENSION SCRIPTS YOU MUST KNOW WHAT YOU ARE DOING WITH RESPECT TO iptables/Netfilter AND SHOREWALL.

The following scripts can be supplied:

  • lib.private -- Intended to contain declarations of shell functions to be called by other run-time extension scripts. See this article for an example of its use.

  • compile -- Invoked by the rules compiler early in the compilation process. Must be written in Perl.

  • init -- invoked early in “shorewall start” and “shorewall restart

  • initdone -- invoked after Shorewall has flushed all existing rules but before any rules have been added to the builtin chains.

  • start -- invoked after the firewall has been started or restarted.

  • started -- invoked after the firewall has been marked as 'running'.

  • stop -- invoked as a first step when the firewall is being stopped.

  • stopped -- invoked after the firewall has been stopped.

  • clear -- invoked after the firewall has been cleared.

  • tcclear -- invoked to clear traffic shaping when CLEAR_TC=Yes in shorewall.conf.

  • refresh -- invoked while the firewall is being refreshed but before the blacklst chains have been rebuilt.

  • refreshed -- invoked after the firewall has been refreshed.

  • maclog -- invoked while mac filtering rules are being created. It is invoked once for each interface having 'maclist' specified and it is invoked just before the logging rule is added to the current chain (the name of that chain will be in $CHAIN).

  • isusable -- invoked when Shorewall is trying to determine the usability of the network interface associated with an optional entry in /etc/shorewall/providers. $1 is the name of the interface which will have been determined to be up and configured before the script is invoked. The return value from the script indicates whether or not the interface is usable (0 = usable, other = unusable).

    Example:

    # Ping a gateway through the passed interface
    case $1 in
        eth0)
            ping -c 4 -t 1 -I eth0 206.124.146.254 > /dev/null 2>&1
            return
            ;;
        eth1)
            ping -c 4 -t 1 -I eth1 192.168.12.254 > /dev/null 2>&1
            return
            ;;
        *)
            # No additional testing of other interfaces
            return 0
            ;;
    esac

    Caution

    We recommend that this script only be used with ADMINISABSENTMINDED=Yes.

    The firewall state when this script is invoked is indeterminate. So if you have ADMINISABSENTMINDED=No in shorewall.conf(8) and output on an interface is not allowed by routestopped(8) then the isuasable script must blow it's own holes in the firewall before probing.

  • save -- This script is invoked during execution of the shorewall save and shorewall-lite save commands.

  • restored -- This script is invoked at the completion of a successful shorewall restore and shorewall-lite restore.

  • findgw -- This script is invoked when Shorewall is attempting to discover the gateway through a dynamic interface. The script is most often used when the interface is managed by dhclient which has no standardized location/name for its lease database. Scripts for use with dhclient on several distributions are available at http://www.shorewall.net/pub/shorewall/contrib/findgw/

If your version of Shorewall doesn't have the file that you want to use from the above list, you can simply create the file yourself. You can also supply a script with the same name as any of the filter chains in the firewall and the script will be invoked after the /etc/shorewall/rules file has been processed but before the /etc/shorewall/policy file has been processed.

The following table indicate which commands invoke the various scripts.

scriptCommands
clearclear
compilecheck, compile, export, load, refresh, reload, restart, restore,start
continue 
initload, refresh, reload, restart restore, start
initdonecheck, compile, export, refresh, restart, start
isusablerefresh, restart, restore, start
maclogcheck, compile, export, refresh, restart, start
refreshrefresh
refreshedrefresh
restoredrestore
savesave
startload, reload, restart, start
startedload, reload, restart, start
stopstop, clear
stoppedstop, clear
tcclearload, reload, restart, restore, start

There are a couple of special considerations for commands in extension scripts:

  • When you want to run iptables, use the command run_iptables instead. run_iptables will run the iptables utility passing the arguments to run_iptables and if the command fails, the firewall will be stopped (or restored from the last save command, if any). run_iptables should not be called from the started or restored scripts.

  • If you wish to generate a log message, use log_rule_limit. Parameters are:

    • Log Level

    • Chain to insert the rule into

    • Chain name to display in the message (this can be different from the preceding argument — see the Port Knocking article for an example of how to use this).

    • Disposition to report in the message (ACCEPT, DROP, etc)

    • Rate Limit (if passed as "" then $LOGLIMIT is assumed — see the LOGLIMIT option in /etc/shorewall/shorewall.conf)

    • Log Tag ("" if none)

    • Command (-A or -I for append or insert).

    • The remaining arguments are passed "as is" to iptables

  • Many of the extension scripts get executed for both the shorewall start and shorewall restart commands. You can determine which command is being executed using the contents of $COMMAND.

    if [ $COMMAND = start ]; then
       ...

Compile-time vs Run-time Scripts

Shorewall runs some extension scripts at compile-time rather than at run-time.

The following table summarizes when the various extension scripts are run:

Compile-timeRun-time
compileclear
initdoneinit
maclogisusable
Per-chain (including those associated with actions)start
 started
 stop
 stopped
 tcclear
 refresh
 refreshed
 restored

Compile-time extension scripts are executed using the Perl 'eval `cat <file>`' mechanism. Be sure that each script returns a 'true' value; otherwise, the compiler will assume that the script failed and will abort the compilation.

Each compile-time script is implicitly prefaced with:

package Shorewall::User;

Most scripts will need to begin with the following line:

use Shorewall::Chains;

For more complex scripts, you may need to 'use' other Shorewall Perl modules -- browse /usr/share/shorewall/Shorewall/ to see what's available.

When a script is invoked, the $chainref scalar variable will hold a reference to a chain table entry.

$chainref->{name} contains the name of the chain
$chainref->{table} holds the table name

To add a rule to the chain:

add_rule( $chainref, <the rule> [ , <break lists> ] );

Where

<the rule> is a scalar argument holding the rule text. Do not include "-A <chain name>"

Example:

add_rule( $chainref, '-j ACCEPT' );

The add_rule() function accepts an optional third argument; If that argument evaluates to true and the passed rule contains a --dports or --sports list with more than 15 ports (a port range counts as two ports), the rule will be split into multiple rules where each resulting rule has 15 or fewer ports in its --dports and --sports lists.

To insert a rule into the chain:

 insert_rule( $chainref, <rulenum>, <the rule> );

The log_rule_limit() function works like it did in the shell compiler with three exceptions:

  • You pass the chain reference rather than the name of the chain.

  • The commands are 'add' and 'insert' rather than '-A' and '-I'.

  • There is only a single "pass as-is to iptables" argument (so you must quote that part).

Example:

log_rule_limit(
               'info' ,             #Log Level
               $chainref ,          #Chain to add the rule to
               $chainref->{name},   #Name of the chain as it will appear in the log prefix
               'DROP' ,             #Disposition of the packet
               '',                  #Limit
               '' ,                 #Log tag
               'add',               #Command
               '-p tcp'             #Added to the rule as-is
               );

Note that in the 'initdone' script, there is no default chain ($chainref). You can obtain a reference to a standard chain by:

my $chainref = $chain_table{<table>}{<chain name>};

Example:

my $chainref = $chain_table{filter}{INPUT};

You can also use the hash references $filter_table, $mangle_table and $nat_table to access chain references in the three main tables.

Example:

my $chainref = $filter_table->{INPUT}; #Same as above with a few less keystrokes; runs faster too